AP Biology: Plants - Special Structures


The growing root includes three regions: the root tip, the elongation region, and the maturation region, all of which you need to know for the AP Biology Exam. The root tips and elongation regions are sites of ongoing primary growth. The root apical meristem includes tiny, undifferentiated cells that continually divide and form the zone of cell division. As cell division in the root apical meristem continues, the new cells left behind grow rapidly in length and push the root tip along. As the cells absorb water, elongation occurs. Tiny root hairs form and provide an increased surface are through which water and dissolved minerals can move into the plant.

All roots have an epidermis (outer protective covering), a cortex (a middle region which stores starch and other minerals), and a stele (the inner cylinder which contain xylems and phloems).

Water and minerals travel through the root cortex either by the apoplast pathway (trough porous cell walls) or symplast pathway (through plasmodesmata).

Once water and minerals reach the inner layer of the cortex (endodermis), they must cross the endodermal cells, tightly-packed cells that regulate the selective passage of water and mineral into the vascular tissue, in order to reach the root’s interior.

In the walls of endodermal cells is the Casparian strip, a belt made of a fatty tissue called suberin, which blocks the movement of water and minerals between the endodermal cells.


Image 1: Root growth


Leaves play an important role in photosynthesis. However, leaves may be modified for other purposes.

- Leaves can be modified to form spines, as in a cactus. This adaptation is great for protection and preventing water loss.
- Leaves can be adapted for water storage. Fleshy leaves allow plants to survive particularly hard environments where the water supply is intermittent or undependable.
- Leaves can also be modified to trap prey. Insectivorous plants have specialized leaves that digest insects. Because thy grow in soils deficient of essential nutrients, especially nitrogen, these plants rely on insects.

Flowering Plants

Flowering plants have several organs:


Image 2: Organs of a flower


The male parts collectively are called the stamen, and the female parts are called the pistil. The sepal are the green, leaf-like structures that cover and protect the flower, while the brightly colored petals attract potential pollinators.


The stamen consists of the anther and the filament. The anther produces pollen grains, called microspores, and are the plant’s male gametophytes (sperm cells). Pollen grains are produced and released into the air. The filament is a thin stalk that holds up the anther.


The pistil includes three structures: the stigma, style, and ovary. The stigma is the sticky portion of the pistil that captures the pollen grains. The style is a tube-like structure that connects the stigma with the ovary.

The ovary is where fertilization occurs. Within the ovary are ovules, which contain megaspores, the plant’s female gametophytes. The megaspores undergo meiosis to produce eight female nuclei, including one egg nucleus and two polar nuclei. In a fertilized plant, the ovary develops into a fruit. For example, apples, pears, and oranges are all fertilized ovaries of flowering plants.

AP Biology Study Tip

Draw diagrams of roots, leaves, and flowers. Label the structures and their functions.


Image 1: http://www2.puc.edu/Faculty/Gilbert_Muth/botglosr.htm

Image 2: http://www.aces.uiuc.edu/vista/html_pubs/PLBREED/pl_breed.html

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